Wavelength conversion device

Abstract

This wavelength conversion device comprises: optical waveguides (101a), (101b) (input optical waveguide (111')), an optical branching unit (151), two arm waveguides (104a), (104b) (array optical waveguide (114')) having different lengths, an optical multiplexing unit (103), and two optical waveguides (105a), (105b) (output optical waveguide (115')). An optical waveguide core of the optical branching unit (151) is composed of an optical nonlinear material, and periodic polarization inversion is formed in a slab optical waveguide part (optical waveguide core) of the optical branching unit (151).

Description

Wavelength conversion device

[0001] The present invention relates to a wavelength conversion device.

[0002] (The Importance of Wavelength Conversion Technology and PPLN) Wavelength conversion technology is attracting attention for applications requiring wavelength ranges that cannot be directly output by semiconductor lasers, or high-power light that cannot be obtained by semiconductor lasers even in wavelength ranges that can be output. Wavelength conversion devices can be made by using optical crystals that have second-order optical nonlinear effects.

[0003] Representative optical crystals include, for example, lithium niobate (LiNbO), potassium niobate (KNbO), lithium tantalate (LiTaO), and potassium titanyl phosphate (KTiOPO). In particular, optical waveguides using periodically poled lithium niobate (hereinafter referred to as PPLN) are devices that can achieve high wavelength conversion efficiency by increasing optical intensity and utilizing quasi-phase matching technology. PPLN is used in optical signal wavelength conversion in optical communications, optical processing, medicine, bioengineering, and other fields, and is expected to be used in a wide range of optical wavelengths from the ultraviolet to the terahertz region.

[0004] (Applications of PPLN to optical communication devices and quantum optical devices) Furthermore, PPLN can be used to fabricate optical parametric amplifiers (OPAs) and pump light generators that constitute phase-sensitive amplifiers (PSAs) capable of low-noise optical amplification. For this reason, PPLN is being considered for use as a device that can achieve high-gain, low-noise optical amplification characteristics and play an important role in the next-generation field of optical fiber communications.

[0005] In the field of quantum computing, an optical waveguide using PPLN can be inserted into a fiber ring resonator and used as a parametric oscillator. Regarding this configuration, it has been reported that an optical coherent Ising machine device has been realized, and that it can perform large-capacity calculations at higher speeds than conventional computers.

[0006] In the field of quantum optics, a PPLN optical waveguide has been used to generate a quantum entangled photon pair called squeezed light by spontaneous parametric down-conversion (SPDC), which eliminates one photon from the pump light and generates two photons (photon pairs) that satisfy the law of energy conservation with the lost photon. This squeezed light is expected to be used in quantum computing, such as quantum teleportation, and quantum cryptography.

[0007] Furthermore, in the field of quantum optical sensing, for example, by using squeezed light, which compresses fluctuations in vacuum field noise, for measurement, it is possible to achieve detection sensitivity below the shot noise (vacuum field noise), which is the sensitivity limit in the field of quantum sensing, and it is expected that optical interference measurement methods with higher sensitivity than conventional ones will be realized.

[0008] (Optical Waveguide-Type Wavelength Conversion Element Made of Optical Crystal, Such as LiNbO3) Next, a wavelength conversion element using an optical crystal, such as the above-mentioned LiNbO3, is described in, for example, Patent Document 1. Patent Document 1 discloses an example of fabricating a ridge-type optical waveguide. Patent Document 1 describes fabricating a wavelength conversion element by bonding a first substrate made of a nonlinear optical crystal having a periodically poled structure to a second substrate having a refractive index smaller than that of the first substrate, in order to improve the light confinement effect in the ridge-type optical waveguide.

[0009] Furthermore, Patent Document 1 describes that in order to avoid cracks due to deterioration of the adhesive or temperature changes, a nonlinear optical crystal of the same type as that of the first substrate is used as the second substrate, and the first substrate and the second substrate are diffusion-bonded by applying heat. In order to further improve the performance of these technologies, it is important to realize a wavelength conversion device with higher wavelength conversion efficiency.

[0010] Patent No. 3753236

[0011] (Explanation of Conventional Wavelength Converter) As shown in Patent Document 1 and the like, in general, when manufacturing a wavelength conversion element having an optical waveguide structure that satisfies the quasi-phase matching condition, an optical nonlinear crystal having a large optical nonlinear constant (susceptibility) is often used as the material used for the optical waveguide core. Examples of optical nonlinear crystals include lithium niobate (LiNbO3), potassium niobate (KNbO3), lithium tantalate (LiTaO3), and potassium titanyl phosphate (KTiOPO4).

[0012] However, in the case of optical nonlinear crystals such as those described above, a very large applied electric field is required to form a periodically poled structure. To achieve this, a very large localized electric field must be formed by applying a large voltage after forming a finely structured metal electrode pattern, necessitating a complex fabrication process for the finely poled periodically poled structure. Thus, at present, it is difficult to form an electrode structure for applying an electric field after processing the optical waveguide core into its shape.

[0013] (Explanation of a conventional manufacturing process of a wavelength conversion element) A conventional manufacturing process of a wavelength conversion element generally includes the following process steps.

[0014] (Step 1) A periodically poled (QPM) structure is formed in advance in the material of the optical waveguide core layer. Specifically, a high electric field in a specific direction is applied to the entire surface of the planar optical waveguide core layer material to align the entire dielectric polarization domain. Next, a polarization inversion electrode pattern is created using a photomask pattern and photolithography process that matches the polarization inversion structure of the desired design value, and a polarized structure is formed within the uniform dielectric polarization described above by applying a high electric field. After this, the photoresist and electrode film are removed to form the periodically poled structure.

[0015] (Second step) The substrate is bonded to a substrate material having a lower refractive index than the optical waveguide core at the wavelength of light to be used. Specifically, the bonding is performed by thermal bonding, corona discharge, or the like using a flat, mirror-polished bonding surface.

[0016] (Third step) Using a grinding or polishing device, the resultant is processed into a wafer shape that can be used in a later photo process such as patterning using a photoresist. At this time, the thickness of the optical waveguide core layer is processed to match the thickness of the optical waveguide core that will be processed later.

[0017] (Step 4) After patterning the optical waveguide core layer into an optical waveguide core shape using a photoresist or the like, the optical waveguide core is formed by dry etching, dicing, proton exchange, or the like to produce a wavelength conversion element having an optical waveguide structure with a periodic polarization inversion structure.

[0018] (Explanation of thickness error and processing error of conventional wavelength conversion element) As mentioned above, since the optical waveguide core is fabricated after the periodic polarization inversion structure is fabricated in advance, processing errors occur when thinning the core layer and when processing the optical waveguide core. These processing errors cause fluctuations in the effective refractive index of the optical waveguide at each optical wavelength of the signal light, pump light, and idler light due to the processing errors. This causes variations in the optical characteristics, such as the optical spectral distribution of the wavelength-converted light of the final wavelength conversion element.

[0019] In other words, due to the thickness error of the wavelength conversion element described above, the phase matching conditions include some error, which causes a decrease in wavelength conversion efficiency such as optical difference frequency generation, and causes the center wavelength of the optical characteristics of the wavelength conversion element to deviate from the design value.

[0020] Furthermore, since these thickness errors are discovered after processing, the polarization reversal periodic structure of the optical waveguide is already determined when the optical waveguide is processed, so in reality, it is not possible to make corrections to correct processing errors such as thickness errors.

[0021] Here, the effective refractive index of the optical waveguide core can be set to a desired value by the following method. First, a photomask pattern is created in which the core width of the optical waveguide core is width-modulated to match variations in thickness error, and the core width of the optical waveguide core is modulated using techniques such as photolithography and dry etching. This configuration makes it possible to correct the effective refractive index of the optical waveguide core to a certain extent.

[0022] However, in reality, there is a limit to how much the effective refractive index of the optical waveguide core can be set to a desired value due to processing errors that occur during the processing of the optical waveguide core. Furthermore, fluctuations in the core width increase the optical loss of the optical waveguide, reducing the optical intensity of the control light and, as a result, reducing the efficiency of the wavelength conversion device.

[0023] (Explanation of Optical Propagation Mode in Optical Waveguide) In addition, in a wavelength conversion element, it is preferable that the optical confinement mode during optical propagation of signal light and pump light is as single as possible. In addition, in order to satisfy the quasi-phase matching condition, it is desirable that the optical confinement mode propagating through the core of the optical waveguide is as single-mode propagation as possible.

[0024] In direct optical coupling using optical fibers (butt-joint optical coupling) or optical coupling using spatial optical systems such as lenses, single-mode propagation with a simple Gaussian distribution is more likely to achieve highly efficient optical connections in order to reduce optical coupling loss at the input and output of light to wavelength conversion elements. When multiple modes of light propagate through an optical waveguide, the effective refractive index of each mode varies slightly, which also deviates from the phase matching conditions during wavelength conversion, preventing the element from functioning as a highly efficient wavelength conversion element.

[0025] (Issues of the photorefractive effect and optical damage) However, in wavelength conversion elements, when the optical mode during propagation of the signal light and pump light is single and the confinement effect is high, and the optical intensity becomes strong, a phenomenon called the photorefractive effect occurs, in which a refractive index change is induced in the crystal according to the spatial intensity distribution of the incident light. When this phenomenon occurs, there is a problem that the effective refractive index and the periodic polarization reversal structure of the optical waveguide of the wavelength conversion element change, deteriorating the optical characteristics of the wavelength conversion element.

[0026] Furthermore, in optical nonlinear optical crystal materials such as LiNbO3, a localized light-induced refractive index change called optical damage occurs when high-intensity light such as a short-wavelength laser is irradiated, and a phenomenon called laser damage occurs when a very large internal electric field is locally induced within the crystal by localized laser irradiation with a strong optical energy density, causing dielectric breakdown. Generally, the above-mentioned optical damage and laser damage are often collectively referred to as optical damage.

[0027] (Explanation of temperature control of conventional wavelength conversion element) Of course, by using a temperature control element such as a Peltier element for the wavelength conversion element to perform high-precision temperature control, it is possible to control, for example, the center wavelength of the optical wavelength of idler light generation to some extent.

[0028] Here, it is possible to correct the effective refractive index to some extent on average for thickness errors in the wavelength conversion element, but to correct all local thickness variations, it is necessary to control all the temperatures of the local optical waveguide cores of the wavelength conversion element, which makes the temperature control elements and control circuits complex, making detailed control difficult.

[0029] In reality, the entire wavelength conversion element is not always at exactly the same temperature, and there is a temperature distribution inside the wavelength conversion element due to heat exchange with the temperature control element, the temperature difference between the ambient temperature and the wavelength conversion element, the state of radiant heat from the wavelength conversion element and the surroundings of the mounting structure, etc. For this reason, even if the effective refractive index of the optical waveguide core of the wavelength conversion element has a completely consistent unique value, variations in the effective refractive index occur within a certain range.

[0030] (a) (Problem of distortion of spectral waveform due to fluctuations in the shape of the optical waveguide or the polarization structure) For example, in a wavelength conversion element, the wavelength spectrum of the second harmonic generated by an ideal periodic polarization inversion structure ideally exhibits a Sinc function distribution.

[0031] However, as mentioned above, in conventional wavelength conversion elements, the difference frequency generation by the ideal periodically poled structure is disturbed by thickness errors, processing errors, temperature distribution variations, etc. that occur in the conventional structure, resulting in difference frequency generation with a distorted spectral waveform including large side peaks. The above-mentioned variation in the effective refractive index of the periodically poled structure can be corrected to some extent by improving processing accuracy, etc. However, this can only be achieved by maintaining manufacturing accuracy at a certain rate.

[0032] (b) (The problem of needing another optical element to achieve further narrowband (narrower linewidth)) Therefore, in order to stably reduce the disturbance in the wavelength spectrum of the wavelength conversion element obtained, it is more practical to narrow the difference frequency generation peak by using a stable narrowband wavelength optical filter and limiting the transmission band of the wavelength spectrum of the difference frequency generation light obtained with the narrowband optical filter, rather than attempting to correct the processing shape error of the waveguide having quasi-phase matching or local temperature control as described above.

[0033] For example, as described above, by narrowing the bandwidth of the peak of the optical spectrum of the difference frequency generated light, the second harmonic (SHG light) of the transmitted signal light can be narrowed in bandwidth, and then optically amplified and used as pump light, thereby enabling PSA amplification of phase-locked signal light.

[0034] In other words, while reducing processing errors and temperature control errors is an important technology, when fabricating an integrated wavelength conversion device, it is extremely important to provide a certain degree of margin for processing errors in the optical waveguide of the second-order nonlinear material and for local temperature control technology, to control the spectral shape of the generated frequency-generated light itself, and to suppress distortion and disturbance of the generated idler light.

[0035] The technology that can control the optical spectrum width and peak shape of difference frequency generated light is essential for improving the efficiency of wavelength conversion elements using second-order optical nonlinear materials with quasi-phase matching. However, at present, it depends on controlling the fabrication precision, temperature distribution, and control of the optical waveguide with quasi-phase matching.

[0036] (c) (The problem that the PPLN wavelength conversion element itself does not have a multiplexing / demultiplexing function) As mentioned above, in a wavelength conversion device using a general second-order nonlinear material, only pump light is input into an optical waveguide core using a second-order nonlinear material to generate second-harmonic light, doubled wavelength light, etc. Alternatively, it is possible to input two types of light, signal light and pump light, to generate idler light.

[0037] However, it is very difficult to provide a wavelength conversion element using a simple linear optical waveguide, such as the wavelength conversion element described above, with the function of multiplexing or demultiplexing signal light and pump light with different wavelengths, and therefore, in addition to the wavelength conversion element, it is necessary to optically connect a multiplexer or demultiplexer for each signal light and pump light. Furthermore, when optically connecting an optical multiplexer or demultiplexer to a wavelength conversion element, variations in optical characteristics such as optical connection loss and propagation loss occur depending on the wavelength of each signal light and pump light, and when the variations in optical characteristics become large, it becomes necessary to adjust the optical intensity.

[0038] (d) (Issues in Generating Quantum Entangled Light (Squeezed Light)) First, the generation of squeezed light in an optical waveguide type wavelength conversion element using a second-order optical nonlinear material will be described.

[0039] (Explanation of quantum entangled light (squeezed light) generation) When SHG light with a frequency of 2ω is incident on the central channel optical waveguide of the input optical waveguide, spontaneous parametric down-conversion light (SPDC light) with a frequency of ω is generated due to the periodic polarization inversion formed in the slab optical waveguide of the demultiplexer, and one photon disappears from the pump light, generating two photons (photon pair) that satisfy the law of energy conservation with this photon, thereby generating squeezed light.

[0040] A squeezed state (squeezing) is a state in which one of the conjugate quantities suppresses fluctuations at the expense of increasing the other. A squeezed state allows the redistribution of the magnitude of uncertainty between the quadrature phase components while maintaining the uncertainty of a coherent state like a laser. For example, shot noise (vacuum field noise) is one of the smallest units of measurement noise in sensing. When a vacuum field is squeezed, for example, it is possible to reduce the fluctuations of one phase component (e.g., the sine component) by allowing the other phase component (e.g., the cosine component) to increase, and even in this state, the overall quantum uncertainty is kept to a minimum.

[0041] The squeezing parameter r (r≧0) is a quantity that indicates the degree of the squeezed state. If the fluctuation of the coherent state is 1, the magnitude of the fluctuation of the squeezed component is e -2r For example, when r = 0.347, e -2r = 0.5, and the fluctuation of this phase component is half of the coherent state (vacuum field level). This is the so-called 3 dB squeezing level. Of course, the magnitude of the other quadrature phase component fluctuation at this time is e 2r and the magnitude of the overall fluctuation (the product of the quadrature component fluctuations) is preserved.

[0042] (Issues with quantum entangled light (squeezed light)) As mentioned above, in the field of quantum sensing, it is possible to use squeezed light, which compresses fluctuations in vacuum field noise, for measurements. This makes it possible, in principle, to reduce shot noise (vacuum field noise), which can be the sensitivity limit in conventional coherent state measurements, and realizes optical interferometry that is more noiseless and has higher sensitivity than conventional methods.

[0043] However, the squeezing level of squeezed light is lost due to state changes caused by propagation loss and the like, so it is desirable to reduce optical loss as much as possible when optically multiplexing and demultiplexing light into each optical waveguide and the like.

[0044] Furthermore, when quantum entangled photon pairs called squeezed light are generated by spontaneous parametric down-conversion (SPDC) using a PPLN optical waveguide, if only one specific wavelength is to be used, a wavelength conversion element using a linear PPLN optical waveguide can be used.

[0045] However, in the case of wavelength-multiplexed squeezed light applications in which squeezed light is generated by setting channel widths of multiple wavelengths for a certain wavelength range and used in quantum teleportation or quantum computing, a wavelength conversion element using a linear PPLN optical waveguide alone will propagate the squeezed light of all wavelengths in a mixed state through the optical waveguide, and therefore the squeezed light must be demultiplexed by an optical demultiplexer into channel widths of each of the multiple wavelengths before being used in quantum teleportation or quantum computing.

[0046] Here, the squeezed level, which indicates the degree of quantum entanglement of squeezed light, decreases with optical loss, so optical loss caused by optical spectrometers, etc. cannot be ignored. Therefore, when introducing an optical spectrometer separately, it is important to reduce the increase in optical loss due to its introduction.

[0047] As described above, in the conventional technology, in order to use multiple signal lights and multiple idler (wavelength-converted) lights in a wavelength conversion element, it is necessary to optically couple them to an optical waveguide-type wavelength conversion element using a separate optical multiplexer / demultiplexer. However, this method has problems such as increased optical loss and an increase in the overall mounting volume of the optical module.

[0048] The present invention has been made to solve the above problems, and aims to enable a wavelength conversion device to use multiple signal lights and multiple wavelength-converted lights without increasing optical loss or increasing the overall mounting volume of the optical module.

[0049] The wavelength conversion device according to the present invention is composed of a waveguide-type optical interferometer circuit configured on the same plane, and includes an input optical waveguide, an optical branching section, an optical multiplexing section, an output optical waveguide, and an arrayed optical waveguide consisting of a plurality of optical waveguides of different lengths sandwiched between the optical branching section and the optical multiplexing section, and at least one of the optical branching section and the optical multiplexing section is composed of an optical waveguide core made of a second-order optical nonlinear material having a periodically poled structure.

[0050] A wavelength conversion device according to the present invention is composed of a waveguide-type optical interferometer circuit configured on the same plane, and includes a first input / output optical waveguide, a first optical multiplexing / branching section, a second optical multiplexing / branching section, a second input / output optical waveguide, and an arrayed optical waveguide consisting of a plurality of optical waveguides of different lengths sandwiched between the first optical multiplexing / branching section and the second optical multiplexing / branching section, the first optical multiplexing / branching section and the second optical multiplexing / branching section being composed of an optical waveguide core made of a second-order optical nonlinear material having a periodically poled structure, and the first input / output optical waveguide and the second input / output optical waveguide being composed of a central channel optical waveguide at a position where the diffraction angle is 0 degrees, and a plurality of optical waveguides having a center of symmetry at the connection position of the central channel optical waveguide.

[0051] As described above, according to the present invention, at least one of the optical branching section and the optical multiplexing section is constructed from an optical waveguide core made of a second-order optical nonlinear material having a periodically poled structure, so that a plurality of signal lights and a plurality of wavelength-converted lights can be used in the wavelength conversion device without increasing optical loss or increasing the overall mounting volume of the optical module.

[0052] FIG. 1 is a diagram showing the schematic configuration of a Mach-Zehnder interferometer-type optical multiplexer / demultiplexer. FIG. 2 is a diagram showing the schematic configuration of an arrayed waveguide grating (AWG). FIG. 3 is a characteristic diagram showing an example of a transmittance spectrum from the central input port to the central output port of an AWG made of a silica-based PLC. FIG. 4 is an explanatory diagram for explaining the operating principle of an arrayed waveguide grating (AWG). FIG. 5 is a diagram showing the schematic configuration of a wavelength converter in which a wavelength converter 141 with a linear optical waveguide structure and a Mach-Zehnder interferometer-type optical multiplexer / demultiplexer 142 are optically connected in series. FIG. 6 is a diagram showing the configuration of a wavelength converter in which a wavelength converter 141 with a linear optical waveguide structure and an arrayed waveguide grating (AWG) 143 are optically connected in series. FIG. 7 is a diagram showing the configuration of a wavelength converter according to a first embodiment of the present invention. FIG. 8 is a diagram showing the configuration of another wavelength converter according to the first embodiment of the present invention. FIG. 9 is a diagram showing the configuration of an optical branching unit 153 of a wavelength converter according to a second embodiment. FIG. 10 is a configuration diagram showing the configuration of another optical branching unit 153′ of a wavelength conversion device according to a second embodiment of the present invention. FIG. 11 is a configuration diagram showing an example of the configuration of a wavelength conversion device using a second-order nonlinear optical medium such as a conventional PPLN waveguide. FIG. 12 is a characteristic diagram showing the difference frequency generation (DFG) band in optical parametric amplification. FIG. 13 is an explanatory diagram explaining the operating principle of an arrayed waveguide grating (AWG). FIG. 14 is a configuration diagram showing the configuration of an optical wavelength conversion device according to a third embodiment of the present invention. FIG. 15 is a characteristic diagram showing the difference frequency generation band of the optical wavelength conversion device according to the third embodiment. FIG. 16 is a configuration diagram showing the configuration of a wavelength conversion device according to a fourth embodiment of the present invention. FIG. 17 is a characteristic diagram showing the difference frequency generation band of the optical wavelength conversion device according to the fourth embodiment. FIG. 18 is a configuration diagram showing the configuration of a wavelength conversion device according to a fifth embodiment of the present invention. FIG. 19 is a configuration diagram showing the configuration of a wavelength conversion device according to a first embodiment of the present invention. FIG. 20 is a configuration diagram showing the configuration of a wavelength conversion device according to a second embodiment of the present invention. FIG. 21 is a characteristic diagram showing the state of quantum entangled squeezed light in the wavelength conversion device according to the second embodiment of the present invention.

[0053] Hereinafter, a wavelength conversion device according to an embodiment of the present invention will be described.

[0054] First, before describing a wavelength converter according to an embodiment of the present invention, a waveguide-type optical interferometer and a wavelength converter will be described.

[0055] [Explanation of Mach-Zehnder Interferometer (MZI)] First, as an example of a conventional waveguide-type optical interferometer circuit, an outline of a Mach-Zehnder interferometer (MZI) will be explained with reference to Fig. 1. Fig. 1 is a diagram showing the schematic configuration of a Mach-Zehnder interferometer-type optical multiplexer / demultiplexer.

[0056] The MZI has an input optical waveguide 111′ consisting of two optical waveguides 101 a and 101 b, an optical branching section 102 consisting of a directional coupler, an optical multiplexing section 103, an arrayed optical waveguide 114′ consisting of two arm waveguides 104 a and 104 b of different lengths sandwiched between the optical branching section 102 and the optical multiplexing section 103, and an output optical waveguide 115′ consisting of two optical waveguides 105 a and 105 b formed on a planar lightwave circuit (PLC) substrate 100. Signal light input from the input optical waveguide 111′ is branched in the optical branching section 102 into the two arm waveguides 104 a and 104 b of the arrayed optical waveguide 114′. The demultiplexed signal lights propagate through the arm waveguides 104a and 104b, respectively, and are then multiplexed in the optical multiplexing section 103. The lights that have passed through the arm waveguides 104a and 104b interfere with each other and are output to the optical waveguides 105a and 105b of the output optical waveguide 115'.

[0057] When the wavelength used for the signal light is λ, the effective refractive index of the optical waveguide is n, and the difference in length between the two arm waveguides 104 a and 104 b having different lengths is ΔL, the wavelength dependency J(λ) of the transmittance of the Mach-Zehnder interferometer (MZI) of FIG. 1 is expressed by the following equation (1):

[0058] J(λ)=1 / 2×{1+cos[2πnΔL / λ]}...Formula (1)

[0059] From equation (1), in the Mach-Zehnder interferometer type optical multiplexer / demultiplexer of FIG. 1, the wavelength λc at which the transmittance becomes maximum is given by the following equation (2).

[0060] λc=n×ΔL / k... Formula (2)

[0061] Here, k is a positive integer.

[0062] Next, an arrayed waveguide grating (AWG) will be described as another example of a conventional waveguide-type optical interferometer circuit. Fig. 2 is a diagram showing a schematic configuration of an arrayed waveguide grating (AWG).

[0063] In FIG. 2, an input optical waveguide 111, an optical branching section 112, an optical multiplexing section 113, an arrayed optical waveguide 114 consisting of a plurality of waveguides, and an output optical waveguide 115 consisting of a plurality of waveguides are formed on a substrate 100 of a planar lightwave circuit (PLC).

[0064] In the AWG, signal light input from an input optical waveguide 111 is branched into each waveguide of an arrayed optical waveguide 114 in an optical branching section 112, and propagates through each waveguide of different lengths in the arrayed optical waveguide 114. The signal light propagating through the arrayed optical waveguide 114 is multiplexed and interferes with each other in an optical multiplexing section 113, and interference light is output from each waveguide of an output optical waveguide 115.

[0065] Such an arrayed waveguide wavelength multiplexer / demultiplexer has the advantage that it can simultaneously multiplex and demultiplex wavelength-multiplexed light containing many wavelengths by using the interference of multiple light beams propagating through an arrayed waveguide consisting of tens to hundreds of parallel-arranged waveguides, each differing in length by ΔL, and is therefore used as a key device in wavelength-division multiplexed optical communications.

[0066] An example of the transmittance spectrum from the central input port to the central output port of the AWG using the silica-based PLC described with reference to Figure 2 is shown in Figure 3. The transmittance spectrum in Figure 3 shows excellent narrowband characteristics with a transmission band of approximately 1 nm at the peak center of 1545.5 nm.

[0067] (Explanation of the Grating Equation) Next, the operating principle of an arrayed waveguide grating (AWG) will be explained with reference to Fig. 4. In a slab optical waveguide type optical branching unit 131 on the left side of Fig. 4, parallel light of the same phase enters an arrayed optical waveguide 132, and a phase difference given during propagation through the arrayed optical waveguide 132 causes interference in a slab optical waveguide type optical multiplexing unit 133 on the right side, causing diffracted light to propagate through the optical multiplexing unit 133 as interference light having angle dependency.

[0068] The effective refractive index of the arrayed optical waveguide 132 is n c , the effective refractive index of the optical branching unit 131 and the optical multiplexing unit 133 is n s , the diffraction angle is θ, the wavelength is λ, m is the diffraction order (a positive integer), and ΔL is the difference in length between adjacent waveguides of the arrayed optical waveguide 132. The in-phase condition under which the wavefronts output from the respective waveguides of the arrayed optical waveguide 132 are aligned is expressed by the following equation:

[0069] n s dθ+n c ΔL=mλ...Formula (3)

[0070] In equation (3), the wavelength λc at which θ=0 is the wavelength at which the light transmittance in the diffraction order m is maximum, and is given by the following equation.

[0071] λc=n×ΔL / m... Formula (4)

[0072] The wavelength λc is called the central wavelength (fundamental wavelength) and is the wavelength at which the diffraction angle is 0 degrees. When the wavelength is longer than λc, diffraction occurs in the direction of θ>0, and when it is shorter than λc, diffraction occurs in the direction of θ<0. The diffraction order m indicates the wavelength to which the optical path length difference ΔL of the arrayed optical waveguide 132 corresponds. By arbitrarily changing ΔL and m with the length of the waveguide, various optical wavelength filter characteristics can be obtained.

[0073] (Explanation of second-order nonlinear optical effect and phase matching condition) Next, the second-order nonlinear optical effect and phase matching condition will be explained. Generally, when signal light (wavelength: λ1, frequency: ω1) and pump light (wavelength: λ3, wavenumber: ω3) with different wavelengths are input as signal light into a second-order nonlinear optical crystal, wavelength-converted light (wavelength: λ2, frequency: ω2) is generated. This wavelength-converted light wave is called idler light (Ider light).

[0074] From the law of conservation of momentum, the following relationship, which is the phase matching condition, holds:

[0075]

[0076] Consider the case of sum frequency generation ω3 = ω1 + ω2. Since the momentum of a photon is expressed as hk / (2π) using Planck's constant h and angular wave number k, the relationship in equation (6) below holds true according to the law of conservation of momentum.

[0077]

[0078] Therefore, the following equation (7) is obtained.

[0079]

[0080] If the length of the second-order nonlinear optical crystal through which light propagates is L and the propagation direction is the Z direction only, the phase of the nonlinear polarization Pz(ω1 + ω2) changes by exp[i(k1 + k2)Z]. Since the phase of the generated amplitude E(ω3) is exp(ik3 Z), the following equation holds between the two: "exp(ik3 Z) - exp[i(k1 + k2) Z] = exp[i(k3 - k1 - k2) Z] = exp[iΔk Z]...Equation (8)."

[0081] From the above, a phase difference of Δk·L occurs. When this phase difference exceeds π, the phase is inverted, the direction of energy flow is reversed, and a process occurs in which a photon of frequency ω3 is split into a photon of frequency ω1 and a photon of frequency ω2. In this way, the light wave of the sum frequency component that was so painstakingly created begins to decrease. The distance at which the phase is inverted, "Lc = π / (|Δk|)...Equation (9)", is called the coherence length.

[0082] Furthermore, when this phase difference exceeds 2π (i.e., the propagation length of the light exceeds twice the coherence length), the direction of energy flow returns to the original, and it can be seen that the nonlinear polarization Pz increases and decreases in a period twice the coherence length (increase and decrease alternate for each coherence length). Therefore, in order to increase the efficiency of generating wavelength-converted light, the coherence length at which attenuation begins must be made longer than the crystal length through which the light propagates. In particular, the condition Δk = 0 under which there is no wavenumber mismatch is called the phase matching condition, and is the condition for generating wavelength-converted light.

[0083] In this case, when two light waves with frequencies ω1 and ω2 are input into a second-order nonlinear material as described above and idler light of ω2 (=ω1 + ω3) is generated, this is called sum-frequency generation (SFG).On the other hand, when two light waves with frequencies ω1 and ω3 are input into a second-order nonlinear material and idler light of ω2 (=ω3 - ω1) is generated, this is called difference frequency generation (DFG).

[0084] Furthermore, the phenomenon of generating two light waves with frequencies ω1 and ω2 by injecting pump light with a high optical intensity at frequency ω3 is called the optical parametric effect. If we consider the case where all coupled light waves travel in the same direction, the wavenumber mismatch Δk is expressed as "Δk = 2π (n3 / λ3 - n1 / λ1 - n2 / λ2) ... Equation (10)," and the phase matching condition is therefore "n3 / λ3 = n1 / λ1 + n2 / λ2 ... Equation (11)" or "ω1n1 + ω2n2 = ω3n3 ... Equation (12)."

[0085] In the above equation, n1, n2, and n3 are the refractive indices of the second-order nonlinear material through which light of wavelengths λ1, λ2, and λ3 (respective frequencies: ω1, ω2, and ω3) propagates. This means that equation (11) means that the weighted average of n1 and n2, with frequency as the weight, is equal to n3. In particular, in second-harmonic generation, when the polarization of the coupled fundamental wave photons is the same, the phase matching condition is satisfied when the refractive indices of the fundamental wave and the second-harmonic wave are equal. However, in reality, since all materials have refractive index chromatic dispersion, the phase matching condition is not easily satisfied.

[0086] (Explanation of a structure incorporating an interference circuit after a wavelength conversion element) Before describing one embodiment of the present invention, as a further explanation, significant differences from wavelength conversion elements in simple series connection will be described with reference to FIGS. 5 and 6.

[0087] Fig. 5 shows a schematic configuration of a wavelength converter in which a wavelength converter 141 having a straight optical waveguide structure and a Mach-Zehnder interferometer type optical multiplexer / demultiplexer 142 are optically connected in series. Fig. 6 shows a schematic diagram of a wavelength converter in which a wavelength converter 141 having a straight optical waveguide structure and an arrayed waveguide grating (AWG) 143 are optically connected in series.

[0088] It is possible to optically input signal light and pump light using a wavelength conversion element with a periodic polarization reversal structure, and demultiplex the resulting idler light by optically connecting it in cascade to a multiplexing / demultiplexing optical element such as a Mach-Zehnder interferometer type optical multiplexer / demultiplexer 142 or an AWG 143.

[0089] However, since a plurality of signal lights and pump lights are optically connected to one optical waveguide type wavelength converter, the local light intensity becomes strong, making optical damage more likely to occur.

[0090] For this reason, in the first embodiment of the present invention described below, instead of optically connecting an optical waveguide type wavelength conversion element and a multiplexer / demultiplexer in a cascade manner, the wavelength conversion function is integrated into the multiplexer / demultiplexer, thereby suppressing local light intensity and reducing optical loss, and further providing each optical waveguide with the function of demultiplexing multiple wavelengths, or conversely, the function of multiplexing multiple signal lights, thereby realizing a wavelength conversion device that has not been realized until now.

[0091] [Embodiment 1] A wavelength converter according to embodiment 1 of the present invention will be described below with reference to Fig. 7 and Fig. 8. In embodiment 1 of the present invention, a schematic diagram of an MZI type is shown in Fig. 7, and a schematic diagram of an AWG type is shown in Fig. 8. The wavelength converters shown in Fig. 7 and Fig. 8 are configured from a waveguide-type optical interferometer circuit configured on the same plane.

[0092] This wavelength converter includes optical waveguides 101a and 101b (input optical waveguide 111'), an optical branching section 151, two arm waveguides 104a and 104b (arrayed optical waveguide 114') having different lengths, an optical multiplexing section 103, and two optical waveguides 105a and 105b (output optical waveguide 115'). The wavelength converter shown in Figure 8 also includes an input optical waveguide 111, an optical branching section 152, an arrayed optical waveguide 114 consisting of a plurality of waveguides, an optical multiplexing section 113, and an output optical waveguide 115 consisting of a plurality of waveguides.

[0093] In the first embodiment, signal light and pump light are input through optical waveguides 101a and 101b and input optical waveguide 111, and are output through optical waveguides 105a and 105b and output optical waveguide 115. The first embodiment is characterized in that the optical waveguide cores of optical branching units 151 and 152 are made of an optically nonlinear material, and periodic polarization reversals are formed in the slab optical waveguide portions (optical waveguide cores) of optical branching units 151 and 152.

[0094] In this example, signal light and pump light are input through optical waveguides 101a and 101b and input optical waveguide 111, and output through optical waveguides 105a and 105b and output optical waveguide 115, but the propagation direction can be reversed. Also, the optical multiplexing units 103 and 113 can be configured such that the optical waveguide cores are made of an optical nonlinear material and periodic polarization reversals are formed in the slab optical waveguide portions. In other words, in the wavelength conversion device according to the embodiment of the present invention, at least one of the optical branching unit and the optical multiplexing unit is configured with an optical waveguide core made of a second-order optical nonlinear material having a periodic polarization reversal structure.

[0095] However, suppose both are made of optical nonlinear materials and have periodic polarization inversion as described above. Suppose signal light and pump light are input through optical waveguides 101 a and 101 b and input optical waveguide 111, generating idler light that becomes difference frequency light. In this case, optical branching units 151 and 152 generate idler light of difference frequency light, which is propagated through arrayed optical waveguides with different optical path lengths. Optical interference occurs during propagation through optical multiplexing units 103 and 113, and the interference light is optically focused at specific positions (angles) of optical waveguides 105 a and 105 b and output optical waveguide 115. This makes it possible to selectively extract either the signal light or the idler light. In this case, if the phase matching conditions for the periodic polarization inversion in optical multiplexing units 103 and 113 are ignored, the optical multiplexing units function as a simple interference-type optical wavelength filter.

[0096] However, if the phase matching conditions of the periodic polarization inversion in the optical multiplexing section 103 and the optical multiplexing section 113 are designed to match the signal light, it becomes possible to make the specific optical waveguides 105 a, 105 b and the output optical waveguide 115 function as an interference-type optical wavelength filter for the signal light, or to optically amplify the signal light as an optical parametric amplifier (OPA).

[0097] Furthermore, if the phase matching conditions of the periodic polarization inversion in the optical multiplexing unit 103 and the optical multiplexing unit 113 are designed to match the difference frequency (idler) light, the specific optical waveguides 105 a, 105 b and the output optical waveguide 115 can function as an interference-type optical wavelength filter for the difference frequency (idler) light, and can also be used as an optical parametric amplifier (OPA) to amplify light.

[0098] Second Embodiment Next, a wavelength conversion device according to a second embodiment of the present invention will be described. First, the phase matching conditions of quasi-phase matching, which is the principle of wavelength conversion in a second-order optical nonlinear material, will be described in more detail.

[0099] (Explanation of Quasi-Phase Matching) The phase matching conditions in equations (6) and (7) eliminate the wavenumber mismatch Δk=0, but there is a quasi-phase-matched (hereinafter referred to as QPM) method that allows wavenumber mismatch and modulates the nonlinear susceptibility to cancel the effect of phase shift. This is a technique proposed by Armstrong et al. in 1962. This is a technique that achieves quasi-phase matching by using a structure in which the sign of the nonlinear susceptibility is periodically inverted.

[0100] As mentioned above, the nonlinear polarization increases and decreases over a period twice the coherence length. Therefore, by setting the polarization inversion period to twice the coherence length (polarization inversion at coherence length intervals), the nonlinear polarization waves generated from each point are added together without canceling each other out, creating an effect as if the phase mismatch amount were set to zero.

[0101] If the polarization inversion period is Λ, then from equation (4) for the coherence length, we obtain "Λ = 2 Lc...Equation (13)." Consider the case where all coupled light waves travel in the same direction. From equation (10), the wavenumber mismatch is not zero, but "Δk = 2π(n3 / λ3 - n1 / λ1 - n2 / λ2) = 2π / Λ...Equation (14)." Therefore, we obtain "n3 / λ3 - n2 / λ2 - n1 / λ1 - 1 / Λ = 0...Equation (15)." Equation (15) is the phase matching condition for QPM. Note that n3 is the refractive index at wavelength λ3, n2 is the refractive index at wavelength λ2, and n1 is the refractive index at wavelength λ1.

[0102] This QPM method has the advantage that it can use material orientations that produce the maximum component of nonlinear susceptibility, such as second-order nonlinear crystals, and that the operating wavelength range can be set by selecting the inversion period.In addition, by using it as an optical waveguide, it is possible to confine light densely in a narrow region and propagate it over long distances, so highly efficient wavelength conversion has been achieved to date.

[0103] There are also several known methods for fabricating wavelength conversion elements using quasi-phase matching technology. For example, a crystal substrate that exhibits a nonlinear optical effect (hereinafter referred to as a nonlinear optical crystal) is periodically poled, and then a proton-exchange waveguide is fabricated using the periodically poled structure. Similarly, a nonlinear optical crystal substrate is periodically poled, and then a ridge-type optical waveguide is fabricated using a photolithography process and a dry etching process.

[0104] This wavelength conversion element utilizes second harmonic generation (SHG) and optical parametric oscillation using a wavelength conversion element with a QPM method, which has a periodically polarized structure in which the polarization direction of a ferroelectric crystal or a crystal lacking a center of symmetry (has no center of symmetry) is periodically reversed by 180°.

[0105] Generally, the refractive index of a nonlinear optical crystal has wavelength dispersion, so the speeds of the fundamental wave and the second harmonic are not equal, resulting in a phase difference. As a result, the composite wave of second harmonics generated along the optical path within the crystal is a periodic function. Second harmonics generated at each point within the crystal propagate with a phase shift between each harmonic, resulting in a phase difference of π between the generated second harmonic and the second harmonic generated at a distance known as the coherence length Lc. Beyond the coherence length Lc, the intensity of the composite harmonic decreases, fluctuating repeatedly over this period. QPM reverses the phase of the polarization wave generated from the optical nonlinear material every time this period is reached, i.e., it reverses the sign of the nonlinear optical constant d.

[0106] When the periodic polarization inversion period, known as the QPM condition, is set to twice the coherence length Lc, the phase of the second harmonic wave is inverted, correcting the phase of the composite second harmonic wave from the coherence length Lc. As a result, the optical intensity of the generated second harmonic wave is added without decreasing, increasing the amplitude (intensity) of the second harmonic wave and generating second-harmonic light. Due to this feature, the QPM method can utilize the maximum component of the nonlinear optical constant and can also be used with crystals with low birefringence.

[0107] (Explanation of QPM structure in AWG-PPLN) Next, the optical branching section 153 having a periodically poled structure of a second-order optical nonlinear material to be fabricated in the second embodiment will be described with reference to Fig. 9. Note that the wavelength conversion device according to the second embodiment is also composed of a waveguide-type optical interferometer circuit configured in the same plane, similar to the first embodiment described above.

[0108] The optical branching unit 153 of the wavelength conversion device according to the second embodiment is composed of a slab-shaped optical waveguide having an optical waveguide core with a periodically poled structure made of a second-order optical nonlinear material. Signal light input from the input optical waveguide 111 is incident on the optical branching unit 153 at a light incident point 154 of the optical branching unit 153 and propagates through the optical branching unit 153 while spreading in an arc shape in the in-plane direction (into the paper). Therefore, the in-phase plane of the light is formed by connecting lines perpendicular to a straight line 201 that radiates from the light incident point 154. Therefore, the periodically poled distribution structure of the second-order optical nonlinear material in the optical branching unit 153 may be fabricated with a period that satisfies the phase matching condition of Equation (15) along the straight line 201 that radiates from the light incident point 154.

[0109] The structure of the slab-shaped optical branching unit 153 only needs to ensure an area with a light propagation width that allows the propagating light to spread sufficiently in the in-plane direction (toward the paper surface) within the optical branching unit 153. Therefore, it is not necessary for the shape in plan view to be circular as shown in Fig. 9, and various shapes are also acceptable, such as an optical branching unit 153' that has a fan-shaped shape in plan view as shown in Fig. 10.

[0110] 10, the optical distance R (= distance × effective refractive index of optical branching section 153' through which light propagates) from light incidence point 154, which is the core center of the end of the input optical waveguide, to the center point of each core 114a of the arrayed optical waveguide in optical branching section 153' must always be the same for each core 114a of the arrayed waveguide. If this is not the case, differences will occur in the optical phase distances from the center of the end of the input optical waveguide of the in-phase front generated in the arrayed optical waveguide, resulting in phase differences when optically coupled to each core 114a of the arrayed optical waveguide, and errors will be included in the phase differences generated by the different cores 114a, reducing the interference effect expected in the optical multiplexing section.

[0111] (Explanation of Non-Degenerate Parametric Optical Amplification) Next, non-degenerate parametric optical amplification will be described with reference to Fig. 11. Fig. 11 shows an example of the configuration of a wavelength conversion device using a second-order nonlinear optical medium such as a conventional PPLN waveguide.

[0112] Here, a configuration using a first PPLN waveguide 155′ and a second PPLN waveguide 155″ having the same quasi-phase matching conditions will be described. First, fundamental light in the 1550 nm band is generated from a laser light source 156 used in optical communications, and the fundamental light is amplified using an EDFA optical amplifier 157 to obtain sufficient power to obtain a nonlinear optical effect. The amplified fundamental light is input to the first PPLN waveguide 155′, which is a second-order nonlinear optical element, to generate a second harmonic. The second harmonic generated in this way and the signal light are input to the second PPLN waveguide 155″, which is also a second-order nonlinear optical element, to perform nondegenerate parametric amplification.

[0113] As a result, amplified light of the signal light is output. At the same time as the amplified light of the signal light by the second PPLN waveguide 155'', idler (wavelength-converted) light corresponding to the difference in frequency between the signal light and the second harmonic is also output from the second PPLN waveguide 155'' through a difference frequency generation (DFG) process. If only the idler (wavelength-converted) light is extracted from the output side of the second PPLN waveguide 155'', it will function as a wavelength converter, and if only the amplified signal light is extracted, it will function as an optical amplifier.

[0114] Next, optical parametric amplification and difference frequency generation (DFG) bands will be described with reference to FIG. 12 . Here, the difference frequency generation (DFG) process will be used for explanation, but the same applies to the optical parametric amplification process. The fundamental light is a single-wavelength laser light output from a single laser light source. The phase matching band for the second harmonic of the PPLN waveguide is narrower than the phase matching band for difference frequency generation, but the linewidth of the fundamental laser light is significantly wider. Furthermore, FIG. 12(a) shows the phase matching curve for DFG, and FIG. 12(b) shows the phase matching curve for second harmonic generation (SHG).

[0115] As an example, we will describe the wavelength conversion band of a PPLN waveguide when the fundamental wavelength λ0 (frequency: ω0) is 1545 nm and the pump wavelength λ3 (frequency: 2ω0) is 772.5 nm. By inputting pump light and signal light, converted light is generated by difference frequency generation in the PPLN waveguide. For example, if the signal wavelength λ1 (frequency: ω1) is 1540 nm, converted light with a wavelength of 1550 nm is generated by 2ω0 - ω1. Converted light is generated by folding back on the wavelength axis with the fundamental wavelength λ0 as the center.

[0116] In a PPLN waveguide, the quasi-phase matching condition is satisfied among the three waves of pump light, signal light, and converted light. In other words, it has a polarization-inverted structure with an inversion period Λ that satisfies Equation (15). While the converted light is generated in this way, even if the signal light wavelength is changed, the same conversion efficiency can be obtained between the converted light with a frequency of 2ω0-ω1 and the pump light as long as Equation (15) is satisfied. Specifically, for example, if the signal light wavelength λs (frequency: ω1) is 1539 nm, converted light with a wavelength of 1551 nm is generated by 2ω0-ω1. In generating this converted light, the effective refractive indices n1 and n2 also change. However, because n1 decreases by the amount that n2 increases due to material dispersion, Equation (14) can be satisfied even when the signal light wavelength is changed. This has the advantage of achieving a wide wavelength conversion bandwidth, as shown in the upper part of Figure 12.

[0117] The wavelength conversion efficiency η of the converted light generated as described above is expressed as "η = η max [sin(Δk・L / 2) / (Δk・L / 2)] 2 ...Equation (16)" is given by η max is the efficiency when Δk=0, and L is the interaction length, which is equal to the optical waveguide length in the case of an optical waveguide type wavelength conversion element. From this formula for wavelength conversion efficiency η, it can be seen that the wavelength conversion efficiency of a wavelength conversion element is highly dependent on the refractive index dispersion of the optical nonlinear material used. As with the SHG generation of signal light by a wavelength conversion element, when the wavelength of the pump light is changed, the phase mismatch amount Δk changes rapidly due to the refractive index dispersion of the optical nonlinear material used. For this reason, when the interaction length L is about 50 mm, the allowable range of the pump light wavelength is about 0.1 nm, and stable use is only possible in a very narrow band.

[0118] On the other hand, when the signal wavelength changes, the refractive index change of the signal light wavelength and the refractive index change of the idler light, which is the wavelength-converted light, cancel each other out. For this reason, in the case of a wavelength conversion element using a PPLN optical waveguide made of LiNbO3 with an interaction length of 50 mm (= L), for example, the wavelength conversion efficiency is stable over a very wide band of approximately 60 nm.

[0119] Next, the second embodiment will be described in more detail. As shown in Fig. 9, in the second embodiment, signal light and pump light are input through an input optical waveguide 111 and are branched into each waveguide of an arrayed optical waveguide 114 in an optical branching section 153. The signal light propagating through the arrayed optical waveguide 114 is multiplexed and interfered with by the light from the arrayed optical waveguide 114 in an optical multiplexing section (not shown), and interference light is output from each waveguide of an output optical waveguide (not shown).

[0120] In the optical branching unit 153, the signal light and pump light are input at the center of the end of the input optical waveguide, facing the midpoint of the arrangement of the arrayed optical waveguide 114. The optical branching unit 153 is characterized in that the same periodic polarization structure is arranged at a position that is the same optical phase distance from this input point.

[0121] Of course, since the wavelength conversion function as a wavelength conversion element is necessary, as described above, the optical branching section 153 needs to have a polarization inversion domain structure having a periodic polarization inversion period that satisfies the quasi-phase matching condition as shown in equation (15).

[0122] If the core material of the slab optical waveguide of the optical branching section 153 has no refractive index anisotropy and is isotropic, the boundary surfaces of the periodically poled domains are equidistant from the optical input point 153a and are therefore arranged in a concentric circle shape with the optical input point 153a at the center, as shown in Figure 9.

[0123] On the other hand, if the core material of the slab optical waveguide of the optical branching section 153 has refractive index anisotropy, periodic polarization inversion domains will be formed according to the difference in the direction of the anisotropy, and therefore will be arranged in an elliptical shape according to the degree of anisotropy.

[0124] Furthermore, the signal light and the pump light are input to the same input optical waveguide 111, which corresponds to a diffraction angle θ = 0 degrees. This is a state in which the angle tolerance of the phase matching condition of the second-order optical nonlinear material, known as non-critical phase matching, is also widened.

[0125] However, even if the signal light and the pump light are not input to the same input optical waveguide 111, if the signal light and the pump light are input into the optical branching section 153 at an angle that satisfies the relational expression (5) according to the law of conservation of momentum and the phase matching condition is satisfied, optical parametric amplification and wavelength-converted light can be generated.

[0126] It is also important to note that in anisotropic second-order optical nonlinear materials, the wave vector (the direction of wavefront propagation) and the Poynting vector (the direction of light beam = energy propagation) are not parallel, and even if the wavefronts of the fundamental wave and the second harmonic wave coincide, the energy flow directions differ. This phenomenon is called walkoff. While this is not a problem if the beam width is sufficiently wide, when a narrow beam is used, walkoff spatially separates the fundamental wave and the second harmonic SHG wave, limiting the interaction length. When the signal light and pump light are input through different input optical waveguides, it is necessary to form a periodically poled structure that achieves quasi-phase matching, taking into account the angular fluctuations due to walkoff.

[0127] Third Embodiment Next, a wavelength conversion device according to a third embodiment of the present invention will be described. First, the wavelength resolution of an arrayed waveguide grating (AWG) will be described in more detail.

[0128] (Wavelength Resolution of AWG) The operating principle of an arrayed waveguide grating (AWG) will be described with reference to FIG. 13 . First, the wavelength resolution: Δλ of the arrayed optical waveguide 132 will be described. The wavelength resolution of the arrayed optical waveguide 132 is determined by assuming that the fundamental mode of the input optical waveguide 111 is a Gaussian beam, and for simplicity, that the influence of chromatic dispersion of the effective refractive index of the input optical waveguide 111 does not occur, and that the full width at half maximum (FWHM) of the Gaussian beam of the input optical waveguide 111 is approximately equal to the spot size. An optical signal input to the input optical waveguide 111 enters the arrayed optical waveguide 132 via the optical branching unit 131′, exits the arrayed optical waveguide 132, and propagates through the optical multiplexing unit 133′.

[0129] In this process, the wavelength resolution (Δλ) is a value obtained by converting the spatial resolution into wavelength. Therefore, if the distance between adjacent optical waveguides at the input / output end of the arrayed optical waveguide 132 is d and the optical path length difference, which is the difference in length between adjacent optical waveguides of the arrayed optical waveguide 132, is ΔL, the width of the parallel light is expressed as W = 2θ g ・L f Therefore, the wavelength resolution (Δλ) is expressed as follows: Δλ = (2λd) / (πWm) = (λ d) / (πθ g ・L f ・m) ...Equation (17)

[0130] From the above formula, it is possible to either reduce the pitch as in the case of a general diffraction grating, or to increase the focal length L f It can be seen that the wavelength resolution can be improved (Δλ can be reduced) by lengthening the optical path difference ΔL and increasing the diffraction order m in the arrayed optical waveguide 132.

[0131] As mentioned above, from equation (16) for wavelength conversion efficiency η, the wavelength conversion efficiency of a wavelength conversion element depends heavily on the refractive index dispersion of the optical nonlinear material used. Therefore, when the pump light wavelength is changed, the phase mismatch amount Δk changes rapidly due to the refractive index dispersion of the optical nonlinear material used, and the element can only be used stably in a very narrow band. Therefore, narrowing the band of the pump light used is very important. In optical parametric amplification processes using second-order nonlinear optical media and wavelength conversion elements, narrowing the wavelength band of the pump light is very important.

[0132] However, in practice, as described with reference to FIG. 11 , in order to obtain high-intensity pump light in the non-degenerate parametric amplification process using the second PPLN waveguide 155″, a laser light source 156 that emits an independent narrow-linewidth laser is used, and the light is optically amplified by an optical fiber amplifier such as an EDFA optical amplifier 157, after which second harmonic generation (SHG) is generated in the first PPLN waveguide 155′ or the like and used as pump light.

[0133] The addition of noise components during the optical amplification process of the EDFA and the SHG band of second harmonic generation are larger than the wavelength band of the narrow linewidth laser light source, which broadens the linewidth (wavelength band) of the pump light and reduces the wavelength conversion efficiency.

[0134] The third embodiment is characterized in that the linewidth (wavelength band) of the pump light is made into a very narrow-band coherent light by combining the function of an optical wavelength filter with a narrow linewidth (narrow band) of a Mach-Zehnder interferometer or an arrayed waveguide grating (AWG) with second harmonic (SHG) generated light that is generated under the phase matching conditions of quasi-phase matching of a periodically poled structure made of an optical nonlinear material.

[0135] An optical wavelength conversion device according to a third embodiment will be described with reference to Fig. 14. In the third embodiment, a slab-shaped optical branching section 153 having a periodically poled structure made of a second-order optical nonlinear material is connected as an input optical waveguide of an arrayed optical waveguide 114. The wavelength conversion device according to the third embodiment is also composed of a waveguide-type optical interferometer circuit configured on the same plane, similar to the first embodiment described above.

[0136] Incidentally, light output from a laser light source 156 and optically amplified by an EDFA optical amplifier 157 is introduced into the input optical waveguide 111. An optical multiplexing unit 113 is connected to the arrayed optical waveguide 114, and an output optical waveguide 115 is connected to the optical multiplexing unit 113.

[0137] In the third embodiment, the input optical waveguide 111 is connected to the end of the slab optical waveguide of the optical branching section 153 that is symmetrical with respect to the center point of the arrangement of the connection points with the optical waveguides of the arrayed optical waveguide 114 .

[0138] In other words, the optical branching unit 153 is configured to have a central channel optical waveguide at a position where the diffraction angle is 0 degrees. Furthermore, the optical phase distance of the arrayed optical waveguide 114 is set so that when light of a certain optical wavelength is input to this central channel optical waveguide, light of twice the frequency (e.g., second harmonic) can be optically coupled to the central channel optical waveguide at a position where the diffraction angle of the output optical waveguide 115 connected to the optical multiplexing unit 113 is 0 degrees.

[0139] When the optical multiplexing section is also constructed from optical waveguide cores made of a second-order optical nonlinear material having a periodically poled structure, the optical multiplexing section is also configured to have a central channel optical waveguide at a position where the diffraction angle is 0 degrees. Furthermore, the optical phase distance of the arrayed optical waveguide 114 is set so that when light of a certain wavelength is input to this central channel optical waveguide, light of twice the frequency (e.g., second harmonic) can be optically coupled to the central channel optical waveguide at a position where the diffraction angle of the input optical waveguide connected to the optical branching section is 0 degrees.

[0140] Next, the band in the third embodiment will be explained with reference to Fig. 15. In Fig. 15, (a) shows a phase matching curve for DFG, (b) shows a phase matching curve for second harmonic generation (SHG), (c) shows the bandwidth of the narrow wavelength filter using the arrayed optical waveguide 114, and (d) shows the narrowed pump light spectrum.

[0141] As in the above-described second embodiment, in the third embodiment, it is desirable to form a periodically poled structure of the optical nonlinear material so that the connection point between the input optical waveguide 111 and the optical branching section 153 is set as the light incident point, and the second harmonic of the light emitted from the light incident point has a very narrow linewidth. Furthermore, it is desirable to form a boundary surface of the periodically poled structure perpendicular to the in-phase plane of the light emitted from the light incident point.

[0142] The SHG light generated in the optical branching unit 153 is incident on each optical waveguide of the arrayed optical waveguide 114, where a phase difference is added due to the difference in path length between the optical waveguides, and in the optical multiplexing unit 113, the output lights from each arrayed optical waveguide interfere with each other and are output from the output optical waveguide 115. At this time, the phase difference generated by propagation through each arrayed optical waveguide must be designed to interfere with the wavelength of the SHG light, and since the wavelength is usually different from that of laser light, which is the fundamental wave of SHG, wavelengths other than the SHG light cannot be optically coupled to the output optical waveguide 115 while interfering with each other due to the influence of the refractive index wavelength dispersion of the optical nonlinear material.

[0143] Furthermore, since the laser light also passes through the arrayed optical waveguide 114 and enters the slab optical waveguide of the optical multiplexing section 113, if the optical multiplexing section 113 is also made of a second-order optical nonlinear material, like the optical branching section 153, and a periodic polarization reversal structure is also made to match the same phase plane of the laser light, it is possible to further increase the optical intensity of the SHG light.

[0144] As described above, when signal light and high-intensity pump light are input into the wavelength conversion device according to the third embodiment, idler light is generated. If the optical spectrum width of the signal light is large when this idler light is generated, the optical spectrum widths of the resulting signal light, idler light, SHG light, etc. will also be wide. This causes the optical intensity obtained by parametric optical amplification to spread across the wavelength band, reducing the optical intensity of the idler light at the required wavelength and generating crosstalk light with adjacent optical wavelength channels. Therefore, it is important to narrow the linewidth of the optical spectrum of the signal light, the generated idler (wavelength-converted) light, or the pump light, which all cause the optical spectrum width to be widened.

[0145] [Fourth Embodiment] Next, a wavelength conversion device according to a fourth embodiment of the present invention will be described with reference to Fig. 16. In the fourth embodiment, a slab-shaped optical branching section 153 having a periodically poled structure made of a second-order optical nonlinear material is connected to an arrayed optical waveguide 114. Furthermore, an input optical waveguide 111 is connected to an end of the slab optical waveguide symmetrical with respect to the center point of the arrangement of the connection points of the slab optical waveguides of the optical branching section 153. Note that, like the first embodiment described above, the wavelength conversion device according to the fourth embodiment is also composed of a waveguide-type optical interferometer circuit configured on the same plane.

[0146] In the fourth embodiment, a PPLN waveguide 155 is connected to the input optical waveguide 111. Light output from a laser light source 156 and optically amplified by an EDFA optical amplifier 157 is introduced into the PPLN waveguide 155. An optical multiplexing unit 113 is connected to the arrayed optical waveguide 114, and an output optical waveguide 115 is connected to the optical multiplexing unit 113.

[0147] In this example, the output optical waveguide 115 connected to the optical multiplexing section 113 is composed of a central channel optical waveguide at a position where the diffraction angle is 0 degrees, and a plurality of optical waveguides that are symmetrical about the connection position of this central channel optical waveguide.

[0148] In the fourth embodiment, the objective is to optically couple the signal light generated by non-degenerate optical parametric amplification by periodic polarization reversal of the optical branching unit 153 using a second-order optical nonlinear material and the wavelength-converted (idler) light to the output optical waveguide 115 connected to the optical multiplexing unit 113. For this purpose, when the fundamental wavelength λ (frequency: ω) and the pump light wavelength λ (frequency: 2ω) are taken, the optical waveguide (λ c ) are arranged symmetrically around the center of the signal light (λ1, λ2, λ3, λ4, λ ５ , λ6) and each wavelength converted (idler) light are optically coupled with an output optical waveguide 115.

[0149] Next, the operating principle of the wavelength conversion device according to the fourth embodiment will be described with reference to Fig. 17. In Fig. 17, (a) shows a phase matching curve for DFG, (b) shows a phase matching curve for second harmonic generation (SHG), and (c) shows the demultiplexing bands in each optical waveguide of the arrayed optical waveguide 114.

[0150] The optical signal input from the input optical waveguide 111 is expanded by the optical branching unit 153 and is incident as parallel light on each optical waveguide of the arrayed optical waveguide 114. After this, a phase difference is given to the optical signal by the optical waveguides having different optical phase distances, and the signal is incident on the slab optical waveguide of the optical multiplexing unit 113, and the light from each optical waveguide interferes with each other, causing the emitted light to be diffracted.

[0151] The diffraction angle θ of the emitted light at the end of the slab optical waveguide of the optical multiplexing unit 113 is expressed as follows: θ=0,±λ / (n s ・d), ±2・λ0 / (n s ・d), ±3・λ0 / (n s ・d)).

[0152] That is, by designing the pump light wavelength λp (frequency: 2ω0) and the diffraction angle of the fundamental wavelength λ0 (frequency: ω0) to 0 degrees and fabricating a wavelength conversion device, it becomes possible to extract each signal light at a diffraction angle θ<0 and an idler (wavelength-converted) light at a diffraction angle θ>0. Furthermore, since the signal light and the idler (wavelength-converted) light are symmetrically positioned on the spectral axis with the fundamental wavelength λ0 (frequency: ω0) as the center of symmetry, the diffraction angle is also similar, and they are optically coupled to the output optical waveguide 115 at a symmetrical position about the diffraction angle θ=0. By utilizing the optical characteristics of the AWG as described above, it becomes possible to separate the signal light and the idler (wavelength-converted) light into separate optical waveguides and extract them.

[0153] In the fourth embodiment, a fundamental wave light in the 1550 nm band is generated from a laser light source 156, and the fundamental wave light is amplified using an EDFA optical amplifier 157 to obtain sufficient power to obtain a nonlinear optical effect. The fundamental wave light is then incident on a PPLN waveguide 155, which is a second-order nonlinear optical element, to generate a second harmonic wave that is used as pump light.

[0154] Next, the pump light and the signal lights of wavelengths λ1, λ2, and λ3 are multiplexed and optically coupled to the optical branching unit 153 via the input optical waveguide 111. This optically coupled pump light and the signal lights of wavelengths λ1, λ2, and λ3 undergo wavelength conversion by optical parametric amplification and difference frequency generation in the optical branching unit 153, generating optical amplification of the signal lights of wavelengths λ1, λ2, and λ3 corresponding to the DFG phase matching curve shown in Figure 17(a) and idler (wavelength-converted) lights of wavelengths λ4, λ5, and λ6. The generated signal lights and idler lights interfere within the slab optical waveguide of the optical multiplexing unit 113, optically couple at each diffraction angle, and are branched to each of the output optical waveguides 115. At this time, due to the effect of the wavelength filter in the arrayed optical waveguide 114, an optical output is obtained in which optical crosstalk between the optical waveguides of the output optical waveguide 115 is reduced.

[0155] Fifth Embodiment Next, a wavelength conversion device according to a fifth embodiment of the present invention will be described with reference to FIG. 18 . This wavelength conversion device is composed of a waveguide-type optical interferometer circuit configured on the same plane. In this wavelength conversion device, a first optical multiplexer / demultiplexer 153a having a slab shape and a periodically poled structure made of a second-order optical nonlinear material is connected to one end of an arrayed optical waveguide 114. A first input / output optical waveguide 115a is connected to the end of the slab optical waveguide symmetrical with respect to the center point of the array of connection points of the slab optical waveguides of the first optical multiplexer / demultiplexer 153a. A third PPLN waveguide 155a is connected to the first input / output optical waveguide 115a. Light output from a first laser light source 156a and optically amplified by a first EDFA optical amplifier 157a is introduced into the third PPLN waveguide 155a.

[0156] Meanwhile, a second optical multiplexing / branching unit 153b having a slab shape and a periodically poled structure made of a second-order optical nonlinear material is connected to the other end of the arrayed optical waveguide 114, and a second input / output optical waveguide 115b is connected to the second optical multiplexing / branching unit 153b. The second input / output optical waveguide 115b is connected to an end of the slab optical waveguide symmetrical with respect to the center point of the arrangement of the connection points of the slab optical waveguides of the second optical multiplexing / branching unit 153b. A fourth PPLN waveguide 155b is connected to the second input / output optical waveguide 115b. Light output from a second laser light source 156b and optically amplified by a second EDFA optical amplifier 157b is introduced into the fourth PPLN waveguide 155b.

[0157] The wavelength conversion device according to the fifth embodiment is configured to be symmetrical about the arrayed optical waveguide 114, so that in addition to inputting light from one side of the arrayed optical waveguide 114 and outputting it from the other side, it can also input light from the other side of the arrayed optical waveguide 114 and output it from one side.

[0158] For example, each signal light and pump light are optically input through the first input / output optical waveguide 115a where θ = 0, and each signal light and each idler (wavelength-converted) light are extracted through the second input / output optical waveguide 115b where the diffraction angle is other than θ = 0. In the opposite direction, each signal light and pump light are optically input through the second input / output optical waveguide 115b where the diffraction angle is θ = 0, and each signal light and each idler (wavelength-converted) light are extracted through the first input / output optical waveguide 115a where θ = 0.

[0159] According to the fifth embodiment, it is possible to realize a wavelength conversion device having two optical wavelength filter functions using one wavelength conversion device, and it is possible to improve the integration density of the wavelength conversion device.

[0160] [First Embodiment] Next, a more detailed description will be given using an example. First, a first embodiment will be described with reference to FIG.

[0161] First, the fabrication of the wavelength converter of Example 1 will be described. First, the surface of a Mg-doped, Z-cut LiNbO wafer (LN wafer) was immersed in a LiCl aqueous solution, and a high electric field was applied to both sides of the wafer to align the polarization direction in the LN wafer. Next, the polarization period pitch was set at the desired position of the optical branching section 153 so that the signal light wavelength was 1545 nm and the pump light wavelength was 772.5 nm. A resist pattern for QPM polarization inversion for insulation was formed so that polarization inversion domains could be formed with the polarization period. Au metal electrodes were formed on the surface of the LN wafer with the resist pattern formed, and the LN wafer was immersed in a LiCl aqueous solution. A high electric field was applied to both sides of the LN wafer in the opposite direction to the above, thereby fabricating an LN wafer with the desired periodic polarization inversion (QPM) pattern.

[0162] Next, the LN wafer with the QPM pattern was thermally bonded onto the LiTaO wafer, and the surface of the bonded LN wafer was ground and polished to a thickness that matched the QPM period, forming an LN layer on the LiTaO wafer.

[0163] Next, while performing high-precision alignment using alignment marks, a planar pattern of an optical waveguide having the shape of the arrayed optical waveguide 114 shown in Fig. 19 was exposed to light on the surface of the LN layer. Next, using the formed planar pattern as a mask, a high-frequency inductively coupled plasma (ICP) etching device was used to perform plasma etching with a mixed gas of Ar and CF4, thereby fabricating a ridge-shaped PPLN optical waveguide chip having the structure of the arrayed optical waveguide 114.

[0164] After this, a resin protective film was formed to prevent breakage, and the end faces were cut with a dicing saw. After cutting, the resin protective film was washed away and chipped. Next, the end faces of the ridge-shaped LN optical waveguide were anti-reflective coated to produce a wavelength conversion device comprising an input optical waveguide 111, an optical branching section 153, an arrayed optical waveguide 114, an optical multiplexing section 113, and an output optical waveguide 115, each made of LN. The output optical waveguide 115 was designed to have a wavelength of 1545 nm as the center and to output wavelengths at 2 nm intervals on the long-wave and short-wave sides.

[0165] The fabricated wavelength converter was mounted on a temperature-controlled metal mounting plate and optically connected to a polarization-maintaining fiber (PANDA fiber) 159 by position-controlled lens optical coupling.

[0166] First, using a wavelength-tunable LD light source, light of approximately 1540 nm to 1550 nm was input to the wavelength conversion device of Example 1, and the chip temperature was varied to measure the temperature at which signal light of a wavelength of 1545 nm was transmitted through the central waveguide of the output optical waveguide 115.The chip of the wavelength conversion device was mounted on a Peltier element and the temperature was controlled so that this temperature was maintained.

[0167] Thereafter, a laser beam having a wavelength of 1545 nm and a linewidth of 0.1 nm was emitted from the laser light source 156, and this laser beam was optically amplified by the EDFA optical amplifier 157 and input to the PPLN waveguide 155 fabricated in the same manner as the wavelength conversion device of Example 1, generating a pump beam of 772.5 nm, which is the second harmonic, with a high optical intensity of approximately 1 W.

[0168] Furthermore, two signal lights of 1541 nm and 1543 nm were generated by two wavelength-tunable light sources (not shown).

[0169] The generated pump light and signal light were multiplexed by an optical coupler and input to a PANDA fiber 159, and then focused using an objective lens 160 and optically connected to the wavelength converter of Example 1 through alignment control. Meanwhile, output light from the output optical waveguide 115 was also focused using an objective lens 160 and optically connected to a graded index (GI) fiber 161 through alignment control, and then input to an optical spectrum analyzer 158 to measure the optical wavelength spectrum and optical intensity.

[0170] As a result, an optical output with a wavelength of 1545 nm, which is thought to be SPDC pump light, was measured from the central optical waveguide (central channel optical waveguide) of the output optical waveguide 115, which corresponds to a diffraction angle θ = 0 in the wavelength conversion device of Example 1. Furthermore, signal light with a wavelength of 1543 nm was detected from the optical waveguide adjacent to the central optical waveguide of the output optical waveguide 115, and signal light with a wavelength of 1541 nm was detected from the optical waveguide adjacent to that.

[0171] Furthermore, an optical output of 1547 nm, which is thought to be wavelength converted (idler light), was detected from the optical waveguide adjacent to the opposite side of the central optical waveguide, and an optical output of 1549 nm was detected from the optical waveguide further adjacent thereto.

[0172] The adjacent crosstalk in each optical waveguide of the output optical waveguide 115 was approximately 10 dB or more, and output light of approximately several μW could be detected for each wavelength, confirming that the wavelength conversion device having the quasi-phase matching condition and the multiplexing / demultiplexing by the optical waveguide type interferometer of the arrayed optical waveguide 114 were functioning simultaneously.

[0173] Next, Example 2 will be described with reference to Fig. 20. In Example 2, a wavelength converter was also fabricated in the same manner as in Example 1. In Example 2, as shown in Fig. 21, seven wavelengths from λ1 to λ7 were demultiplexed from each optical waveguide of the output optical waveguide 115 at intervals of approximately 0.05 nm, with 1545 nm at the center.

[0174] Light from a laser light source 156 with a wavelength of 1545 nm and a linewidth of 0.1 nm was optically amplified by an EDFA optical amplifier 157 and input into a PPLN waveguide 155 to generate high-intensity pump light of approximately 20 mW at 772.5 nm, which is a second harmonic. This pump light was then input into the wavelength converter of Example 2. As shown by the dotted line in Figure 21, the quantum entangled squeezed light was converted into squeezed light with a certain bandwidth through the spontaneous emission parametric down-conversion (SPDC) process. In this example, the maximum bandwidth was approximately 0.3 nm, and it was confirmed that the wavelength demultiplexing filter function of the wavelength conversion element of Example 2 allowed the squeezed light to be demultiplexed into each of the seven wavelength bands, λ1 to λ7, indicated by the solid lines in the upper diagram of Figure 21.

[0175] The output light from each optical waveguide of the output optical waveguide 115 was multiplexed with a 50:50 beam splitter (HBS: Half (50:50) Beam-splitter) 163 using a laser light source (LD) as a local oscillator (LO). After that, a balanced homodyne detector 164 was used and the free-run squeezed level was evaluated with an optical spectrum analyzer 158, and squeezed light of approximately 1 dB or less was observed.

[0176] As described above, according to the present invention, at least one of the optical branching section and the optical multiplexing section is constructed from an optical waveguide core made of a second-order optical nonlinear material having a periodically poled structure, so that a wavelength conversion device can utilize multiple signal lights and multiple wavelength-converted lights without increasing optical loss or increasing the overall packaging volume of the optical module.

[0177] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0178] 100...substrate, 101a, 101b...optical waveguide, 103...optical multiplexing section, 104a, 104b...arm waveguide, 105a, 105b...optical waveguide, 111, 111'...input optical waveguide, 114, 114'...array optical waveguide, 115, 115'...output optical waveguide, 151...optical branching section, 152...optical branching section.

Claims

1. A wavelength conversion device comprising an optical waveguide type optical interferometer circuit configured in the same plane, including an input optical waveguide, an optical branching section, an optical multiplexing section, an output optical waveguide, and an array optical waveguide composed of a plurality of optical waveguides with different lengths respectively sandwiched between the optical branching section and the optical multiplexing section, wherein at least one of the optical branching section and the optical multiplexing section is composed of an optical waveguide core of a second-order optical nonlinear material having a periodically poled inversion structure.

2. The wavelength conversion device according to claim 1, wherein at least one of the optical branching section and the optical multiplexing section is composed of an optical waveguide core of a second-order optical nonlinear material having a periodically poled inversion structure, and the optical waveguide core of the second-order optical nonlinear material has a periodically poled inversion structure that satisfies the quasi-phase matching condition with equal optical phase distances between the optical waveguide ends and the ends of the array optical waveguide and on the same phase plane.

3. The wavelength conversion device according to claim 1, wherein the optical branching section is composed of an optical waveguide core of a second-order optical nonlinear material having a periodically poled inversion structure, has a central channel optical waveguide at a position where the diffraction angle is 0 degrees, and the optical path distance of the array optical waveguide is set such that light with a double frequency of the wavelength of the light input to the central channel optical waveguide of the optical branching section can be optically coupled to the central channel optical waveguide at a position where the diffraction angle of the output optical waveguide is 0 degrees.

4. The wavelength conversion device according to any one of claims 1 to 3, wherein the output optical waveguide is composed of a central channel optical waveguide at a position where the diffraction angle is 0 degrees and a plurality of optical waveguides having the connection position of the central channel optical waveguide as the center of symmetry.

5. A wavelength conversion device comprising an optical interference circuit of a waveguide type formed in the same plane, including a first input / output optical waveguide, a first optical multiplexer / demultiplexer, a second optical multiplexer / demultiplexer, a second input / output optical waveguide, and an array optical waveguide composed of a plurality of optical waveguides having different lengths sandwiched between the first optical multiplexer / demultiplexer and the second optical multiplexer / demultiplexer, wherein the first optical multiplexer / demultiplexer and the second optical multiplexer / demultiplexer are composed of optical waveguide cores of a second-order optical nonlinear material having a periodically poled inversion structure, and the first input / output optical waveguide and the second input / output optical waveguide are composed of a central channel optical waveguide at a position where the diffraction angle is 0 degrees and a plurality of optical waveguides having a connection position of the central channel optical waveguide as a center of symmetry.

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